Ferritic stainless steels have been used in a wide range of fields, for example, kitchen utensil, household electrical appliances, and electronic equipment. In recent years, extremely low carbon/nitrogen contents and reduction of impurity elements such as P and S have become possible by the improvement of refining technology, and ferritic stainless steels with corrosion resistance and workability improved by adding stabilizing elements such as Nb and Ti (hereinafter referred to as high-purity ferritic stainless steel) have been being used in a wide range of applications. This is because high-purity ferritic stainless steels are more excellent in economic efficiency than austenitic stainless steels containing large amounts of Ni, the price of which has recently soared.
Also in the field of heat-resistant steel that requires oxidation resistance and high-temperature strength, high-purity ferritic stainless steels such as SUS430J1L, SUS436J1L, and SUH21 are standardized (JIS G 4312). SUS430J1L, SUS436J1L, and SUH21, as represented respectively by 19Cr-0.5Nb, 18Cr-1Mo, and 18Cr-3Al, are characterized by addition of rare elements Nb and No or addition of large amounts of Al. Al-containing high-purity ferritic stainless steels represented by SUH21 have excellent oxidation resistance but have problems with workability, weldability, and fabricability associated with low toughness.
Various studies have hitherto been made on the problems of Al-containing high-purity ferrite mentioned above. For example, Patent Document 1 discloses an Al-containing heat-resistant ferritic stainless steel sheet with excellent workability and oxidation resistance including Cr: 13 to 20%, Al: 1.5 to less than 2.5%, Si: 0.3 to 0.8%, and Ti: 3×(C+N) to 20×(C+N), and a process for producing the same. Patent Document 2 discloses a ferritic stainless steel with excellent steam oxidation resistance and thermal fatigue properties including Cr: 8 to 25%, C: 0.03% or less, N: 0.03% or less, Si: 0.1 to 2.5%, Al: 4% or less, and A value, defined as A=Cr+5(Si+Al), in the range of 13 to 60. Such stainless steels disclosed in Patent Documents 1 and 2 are characterized by combined addition of Al and Si with the amount of Al being reduced. Such steels, however, still have a problem with fabricability because Si is an element that decreases steel toughness. Further, the stainless steel disclosed in Patent Document 3 contains Cr: 11 to 21%, Al: 0.01 to 0.1%, Si: 0.8 to 1.5%, Ti: 0.05 to 0.3%, Nb: 0.1 to 0.4%, C: 0.015% or less, and N: 0.015% or less, and 2% or less of W is added as required to obtain high-temperature strength. The stainless steels disclosed in these Patent Documents ensure oxidation resistance and high-temperature strength by reducing the Al content and adding Si or a rare element W.
One possible method for solving the problems described above is to improve oxidation resistance and high-temperature strength using trace elements without relying on high alloying. Conventionally, rare-earth elements are known as a trace element that dramatically improves oxidation resistance. For example, Patent Document 4 discloses adding one or more of rare-earth elements: 0.2% or less, Y: 0.5% or less, Hf: 0.5% or less, and Zr: 1% or less, with their total amount being 1% or less, to a ferritic stainless steel including Cr: 12 to 32% without relying on Si or Al. Further, for high-temperature strength, Patent Document 5 discloses a ferritic stainless steel with excellent high-temperature strength including trace elements Sn and Sb, and a process for producing the same. Most steels disclosed in Patent Document 5 are low-Cr steel including Cr: 10 to 12%, and in the case of high-Cr steel including Cr: more than 12%, V, Mo, and the like are added in combination in order to ensure high-temperature strength. Although the improvement in high-temperature strength is described as an effect of Sn and Sb, there is no discussion or description of the oxidation resistance aimed at by the present invention.
Inventors have hitherto disclosed high-purity ferritic stainless steels with corrosion resistance and workability improved by adding trace amounts of Sn without relying on high alloying of Cr or Mo from the standpoint of resource saving and economic efficiency. The stainless steels disclosed in Patent Documents 6 and 7 are a high-purity ferritic stainless steel including Cr: 13 to 22%; Sn: 0.001 to 1%; C, N, Si, Mn, and P: reduced amount; and Al: in the range of 0.005 to 0.05%; with stabilizing elements Ti and Nb being added as required.
These Patent Documents, however, have not discussed the influence of the addition of trace amounts of Sn and Al on the oxidation resistance and high-temperature strength aimed at by the present invention.
Further, Patent Document 8 discloses a ferritic stainless steel including Cr: 11 to 22%; Al: 1.0 to 6.0%; C, N, and S: reduced amount; and one or more elements selected from the group consisting of Sn: 0.001 to 1.0%, Nb: 0.001 to 0.70%, and V: 0.001 to 0.50% and discloses prevention of evaporation of Cr and/or compounds thereof in an environment where the ferritic stainless steel is exposed to water vapor at high temperature, but does not disclose the effect of addition of Al and Sn on oxidation resistance and high-temperature strength.